The invention relates to a spark plug. Sealing materials are used for tightly fusing a central electrode into an insulator of the spark plug.
A spark plug of the type that defines the species is known, for example, from DE 22 45 403 in which the sealing material for tightly fixing the central electrode in the insulator is comprised of a mixture of a glass as a meltable component and graphite and/or powdered carbon black as an electrically conductive component. Moreover, it is also known to use copper or iron as conductive powdered materials. When the mixture in the insulator is melted, the glass softens as a result of which a contact pin and a central electrode become embedded in the insulator upon fusing. The thermal stability of the fusing, however, is limited because the glass is only sufficiently low viscosity for the fusing at temperatures far above the transformation temperature, while the application limit for the fusing is limited by the transformation temperature of the glass (softening of the fusing, decomposition of the fusing by means of ion transport in the electrical field).
Has the advantage that the sealing material has a higher thermal stability. The proposed metal-ceramic sealing material has a definite melting point and a narrow melting interval, as a result of which the sealing material can be loaded until just below the melting temperature of the metal used. As a result, the distance between the temperature of the fusing and the operating temperature is considerably reduced. This also permits the sealing material to be used in the vicinity of the tip of the insulator base. As a result, it is possible to use the sealing material for nail-shaped platinum electrodes, which have only a slight axial expansion at the tip of the insulator base.
A particularly temperature stable sealing material is achieved if the metallic component is 20 to 40 volume % and the ceramic component is 60 to 80 volume %.